This invention relates generally to processes for producing portland and portland type cement clinker, and more particularly to clinkering processes wherein a fluxing and mineralizing agent is incorporated therein.
Portland cement clinker is typically formed by burning an argillaceous type material such as clay, and a calcareous type material such as limestone, in a kiln at a temperature of about 1450.degree. C. for a period of time, generally in excess of one hour, such that the raw materials react to form new compounds which, when mixed with water, will hydrate and set up as a hard mass. In this reaction, the calcareous or limestone material, e.g., CaCO.sub.3, in the presence of heat gives off CO.sub.2 and becomes CaO or free lime, which then reacts with the silicon dioxide (SiO.sub.2), aluminum oxide (Al.sub.2 O.sub.3), and ferric oxide (Fe.sub.2 O.sub.3) which are the predominant components of argillaceous or clay type materials. The result is that the free lime (CaO) is consumed and reacted and converted to tricalcium silicate or alite, which is the principal component of portland cement clinker. Other calcium-silicate compositions, such as dicalcium silicate, calcium-aluminate compositions, such as tricalcium aluminate (3CaO . Al.sub.2 O.sub.3), and calcium-aluminum-iron compositions and/or solid solutions, such as tetracalcium aluminoferrite (4CaO . Al.sub.2 O.sub.3. Fe.sub.2 O.sub.3), are formed in the reaction and these constitute minor proportions of the resulting clinker composition. A usual, commercial portland cement clinker will contain less than 2.0% by weight free lime, frequently less than 1.0% by weight, and also will contain as much as 55% by weight alite, frequently more than 60% by weight alite.
Portland cement is defined in ASTM C150-74 as a hydraulic cement produced by pulverizing clinker consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground addition. The chemical requirements specify neither a maximum amount of free lime (CaO) nor a minimum amount of alite, i.e., tricalcium silicate (3CaO . SiO.sub.2). The physical requirements are among others, that mortars, the testing of which is specified in ASTM C109-73, made using ASTM Type I portland cement, achieve 3 and 7 day compressive strengths of not less than 1800 psi (pounds per square inch) and 2800 psi, respectively.
In the clinkering process, it is known in the art to add substances in addition to the raw materials, which substances aid in the cement burning, e.g., in the kiln, and these substances are known as fluxing and/or mineralizing agents. As discussed in Lea, The Chemistry of Cement and Concrete, 3rd Edition, 1971, Chemical Publishing Company, on pages 156 and 157, these agents lower the temperature at which liquid or melt is formed (in the burning process) and thus the clinkering temperature. Examples given in Lea are calcium fluoride (CaF.sub.2) and the fluosilicates, such as sodium, magnesium and calcium fluosilicate.
Normally, a flux is a substance which decreases the melting point of the liquid phase, while a mineralizer is a substance which increases the rate of a process and/or reaction occurring within the solid phase, the liquid phase or at the liquid-solid interface. Fluorspar (CaF.sub.2) functions as both a flux and a mineralizer; it lowers the temperature at which liquid is formed, thus reducing the clinkering temperature; and it also increases the reactivity of free lime with intermediary clinker materials.
In addition to lowering the clinkering temperature, fluxes have been used to facilitate clinkering. For example, in 1887, LeChatelier used both calcium chloride and calcium fluoride in his attempts to synthesize pure tricalcium silicate. Further the burnability of difficult-to-clinker mixes, such as low liquid phase compositions, e.g., white portland cement, which is more difficult to burn because of the absence of the iron compounds which partly form the liquid phase, is enhanced using fluxes, such as cryolite (Na.sub.3 AlF.sub.6) which has been used to aid in burning white portland cement clinker where iron contamination is to be avoided.
With the growing scarcity and unavailability of energy, there is an increasing need for new and/or alternative ways of saving and reducing the amount of energy needed for producing cement. For example as reported in Energy Conservation Potential in the Cement Industry, a report by the Portland Cement Association for the Federal Energy Administration, published as Conservation Paper No. 26, June, 1975, today in the U.S., a typical cement kiln is operated at a temperature of approximately 1450.degree. C., and averages about 1600 kcal/kg of cement produced, and so even a modest reduction (100.degree. C.) in the kiln burning temperature could result in energy savings on the order of 150 kcal/kg of clinker. Since fluxing and mineralizing agents reduce the clinkering temperature, there is a corresponding need for improved and/or alternative fluxing and/or mineralizing agents.